A radiologist may frequently need to determine a size of, for example, a nodule using computed tomography (CT) scan images. A nodule may have a smaller size than the size of other structures represented in the CT scan image. For example, FIG. 1A illustrates an exemplary CT scan image 100 of a lung slice. A CT scan image represents an axial or cross-sectional image of a three-dimensional (3D) object, such as a human body. The axial or cross-sectional images represent datasets associated with virtual slices (referred as “slices” hereinafter) of the 3D object. As shown in FIG. 1A, in CT scan image 100, representation 102 is a dot or circle that represents a lung nodule.
Measuring the size of a 3D object, such as long nodule, can provide useful or important information for diagnosis purposes. For example, a radiologist can measure the size of a long nodule at different time points to determine whether the long nodule has grown over time and thus determine whether further medical procedures are required. Determining the size of a 3D object such as a lung nodule can be challenging due to its small size. For example, a nodule may be only a few voxels in diameter and thus difficult to measure. A voxel is a basic element of a 3D image and can have a value on a voxel grid in a 3D image.
Determining the size of a 3D object such as a lung nodule can also be challenging due to the partial volume effect generated by the CT scan system. FIG. 1B illustrates an exemplary voxel grid 120 corresponding to a lung nodule and its surrounding structures. As shown in FIG. 1B, each voxel of voxel grid 120 is represented by a square and is associated with a voxel value. A voxel value can be a density value represents the amount of density of tissues or matters at the voxel's location. A CT scan system can provide the density values of tissues or matters of a lung nodule and its surrounding structures. For example, the air surrounding a lung nodule may have a density value or voxel value of 0 Hounsfield units (HU), while tissues or matters inside the lung nodule may have a density value or voxel value of 100 HU.
In providing the density values of a voxel grid, a CT scan system may also provide values other than 0 HU or 100 HU due to sampling effect and/or mathematical filtering effect. For example, a CT scan system may provide voxel values such as 40, 50, 60, 80, and 97 HU associated with some voxels located between voxels having 0 HU or 100 HU. These voxel values, however, may or may not correspond to tissue or matters of a real structure. As a result, a CT scan system may not provide voxel values that differentiate between the air and tissues or between different tissues. This is referred to as the partial volume effect. The partial volume effect generated by the CT scan system makes it difficult to determine the accurate size of a nodule. For example, in FIG. 1B, it is uncertain whether the size of the nodule corresponds to the size of the voxels with values of 100 HU, the size of the voxels with non-zero voxel values, the size of the voxels having values that are greater than or equal a middle density value between 0 HU and 100 HU, or the size of the voxels having values that are greater than or equal a weighted sum of the non-zero voxels.
The uncertainty or inaccuracy associated with the partial volume effect may become more problematic when the radiologist attempts to determine the variation of the nodule by measuring the size of the nodule using data from a prior CT scan and the size of the nodule using data from a subsequent CT scan. Because both measurements can be inaccurate, the measurement of the nodule variation can be even more inaccurate. With inaccurate variation data, a radiologist can have difficulty in determining whether further medical procedures should be prescribed.